1. Field of the Invention
The present invention relates to the field of bandgap voltage references such as are used in voltage regulators and other integrated circuits.
2. Prior art
At least three prior art bandgap (bipolar technology) reference architectures are well known. All bandgap references use the same underlying principle: they generate a voltage proportional to absolute temperature (V.sub.PTAT) which has a positive temperature coefficient, then they combine this voltage with the base-emitter voltage of a transistor (which has a negative temperature coefficient). When properly combined (e.g. with the proper weighting), the two temperature drifts cancel and one gets a voltage that is fairly independent of temperature. This voltage is typically around 1.23 V and is close to the bandgap voltage of silicon. Hence the name for these references.
FIGS. 1, 2, and 3 show existing bandgap architectures. One of the earliest is shown in FIG. 1. This was used in early 1.23 V reference designs at National Semiconductor (part LM113) and is covered by U.S. Pat. No. 4,249,122. In this circuit, Q2 is operated at 1/10 the current density of Q1. The difference in the V.sub.BE s of Q1 and Q2 will be a V.sub.PTAT. This voltaqe appears across R3 and gets amplified across R2 by the ratio R2/R3. Thus R2's voltage is a V.sub.PTAT voltage. Q3's V.sub.BE is in series with R2's voltage. By proper selection of the parameters, the positive temperature sensitivity of the V.sub.PTAT may be made substantially equal to the negative temperature sensitivity of Q3's V.sub.BE. Under these conditions, the sum of these two voltages will be about 1.23 V. The foregoing reference works well with two terminal 1.23 V references driven by a current source, but doesn't lend itself well to references with higher voltages.
FIG. 2 shows the reference used in the LM185 (also by National Semiconductor) and covered by U.S. Pat. No. 4,447,784. Here Q1 and Q2 are operated at different current densities to give a V.sub.PTAT across R2. The resistors R1, R2 and R3, in series with the emitter-base junction of Q3, give the nominal 1.23 V bandgap reference voltage between the output terminal and the base of Q3. By using a voltage divider consisting of R4 and R5, one can get an output voltage, V.sub.OUT, that is (R4+R5)/R4 times 1.23 V. Thus an arbitrary voltage (&gt;1.23 V) can be generated. Q1 and Q2, in addition to providing the PTAT voltage, also serve as the first stage of an operational amplifier. The remainder of the amplifier is represented by A1.
FIG. 3 shows the Brokaw reference (analog Devices) covered by U.S. Pat. No. 3,887,863. Here the bases of Q1 and Q2 are tied together and the two transistors are operated at different current densities to give a PTAT voltage that appears across R1. By properly adjusting the value for R2, one gets a bandgap voltage from Q2's base-emitter voltage and the sum of the voltages across R1 and R2. This circuit, although functionally equivalent to that of FIG. 2, has a major advantage in that it eliminates the need for Q3 of FIG. 2. Thus it is a simpler circuit, and is now widely used.